When you open a pack, there are two tiers of Shiny Card, with different drop rates. Assuming you don't get a Rare "God Pack", the chance of getting any Tier 1 Shiny Cards is 3.571%.

Pokémon Go hosts a Spotlight Hour every Tuesday. This special event sees spawn rates for one Pokémon boosted greatly. There’s also usually some kind of doubled up bonus, like doubled candy or ...